We examined intra- and extracellular H 2O 2 and NO formation during contractions in primary rat skeletal muscle cell culture. The fluorescent probes DCFH-DA/DCFH (2,7-dichlorofluorescein-diacetate/2,7- dichlorofluorescein) and DAF-2-DA/DAF-2 (4,5-diaminofluorescein-diacetate/4,5- diaminofluorescein) were used to detect H 2O 2 and NO, respectively. Intense electrical stimulation of muscle cells increased the intra- and extracellular DCF fluorescence by 171% and 105%, respectively, compared with control nonstimulated cells (p < .05). The addition of glutathione (GSH) or Tiron prior to electrical stimulation inhibited the intracellular DCFH oxidation (p < .05), whereas the addition of GSH-PX + GSH inhibited the extracellular DCFH oxidation (p < .05). Intense electrical stimulation also increased (p < .05) the intra- and extracellular DAF-2 fluorescence signal by 56% and 20%, respectively. 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